Suction cups are made of a soft, flexible plastic or rubber. When these cups become soiled, it is not only unpleasant looking, it also reduces the function; if suction cups are dirty on the inner, concave area, they will not hold their grip well to a surface. Cleaning these items is not difficult, takes very little time and should completely restore the beauty and function of the cups--unless they are somehow damaged. (Suction cups that have developed cracks may no longer work no matter how clean they are.)
Run a sink full of warm water, and add a few drops of dish soap or hand soap. A tsp. per gallon of water is usually sufficient. Put in the soiled suction cups to soak for 5 to 15 minutes. Soaking, however, is only necessary if they are badly soiled; in many cases you can skip soaking and simply begin washing them right away.
Dampen a clean wash cloth. Use this to gently wash the suction cup. Twist one corner of the cloth to clean out the deepest areas of the cup.
Rinse the cups under warm running water. You should rinse off all the soap residue, or the cups will feel tacky and may not perform as well.
Dry off the cups with a dish towel, and place them on another towel to finish drying completely. The suction cups will be ready to use again right away. If you don't need them immediately, store them in a box or drawer away from sharp objects (that could scratch or puncture them) and away from solvents (which can harm plastics and rubber).